Some 255 million to 275 million metric tons of regulated hazardous waste is reportedly being generated annually in the United States and as much as 80 percent of this regulated hazardous waste is being disposed of in landfills. Additional millions of tons of hazardous waste are probably being disposed of in sanitary landfills by manufacturers who produce less than one metric ton of waste a month and who therefore are exempt from federal regulation.
Much of this landfilled waste will remain hazardous for years or even centuries since landfilling is primarily a containment, not a treatment or detoxification process. At the very least, proper landfill disposal requires accurate record-keeping, increased insurance and trust arrangements, closure and post closure plans, surface and sub-surface monitoring, and caretaking arrangements, perhaps into eternity. Inappropriate disposal of hazardous waste on land creates the risk of contaminating the environment, particularly ground water.
In 1984 a set of far-reaching amendments to the Resource Conservation and Recovery Act of 1976 was enacted in response to both the magnitude and the urgency of the waste disposal problem. Congress has banned land disposal of all hazardous wastes over the ensuing five years. To comply with this new legislation, two major alternatives present themselves: (1) recycle or recovery and reuse and (2) incineration. Recycle or recovery and reuse have the advantage of retrieving value out of the waste. Incineration has the somewhat neutral, but highly desirable attribute of ultimate destruction (except for gaseous products, ash, slag, etc.). The Environmental Protection Agency has undertaken to identify suitable alternatives to land disposal, including treatment, recycling, waste reduction technologies, and long-term storage, with particular emphasis on recycle and treatment of toxic wastes. The landfilling of heavy metals (e.g., arsenic and antimony) and other toxic materials such as polychlorinated biphenyls, dioxin and other halogenated organics will be severely restricted or eliminated.
In some cases otherwise desirable methods of recovery (e.g., distillation for separation and recovery of halocarbons) or incineration cannot be used because of the corrosive and/or highly acidic nature of the waste stream. The spent catalyst from fluorocarbon manufacturing processes is an example of such a waste stream. This catalyst stream may be targeted early by the new legislation since current disposal technology is inadequate. Halocarbon streams from chlorocarbon and fluorocarbon manufacturing processes often contain acidic by-products (e.g., HF and HCl) and heavy metals that either are added as catalysts (e.g., antimony chloride catalysts in fluorocarbon production) or enter the process along with reactants. Arsenic, for instance, is a common impurity in hydrogen fluoride used in fluorocarbon production. However, although present in very small concentrations, arsenic is concentrated as arsenic trichloride in the reactor mass because of the tremendous volume of hydrogen fluoride used.
The acidic and corrosive nature of organic by-product and waste streams from fluorocarbon manufacturing processes plus the presence of heavy metals makes them unsuitable for separation and recovery by distillation. The presence of hydrogen chloride, hydrogen fluoride, and arsenic trichloride results in a multitude of azeotropic combinations that make separation and recovery of useful halocarbons from such spent catalyst by simple or direct distillation impossible. The presence of arsenic trichloride also makes direct incineration impractical since volatile arsenic and antimony chlorides contaminate the solutions used to scrub acids (e.g., HCl and HF) from the incinerator flue gases. Furthermore, inorganic and organic fluorides attack the incinerator firebrick, decreasing the life of the incinerator lining.
Spent antimony catalyst from fluorocarbon manufacturing processes is an extremely hazardous, toxic, and corrosive waste stream, but one with a high potential for the recovery of valuable recyclable and reusable chemicals. Antimony pentachloride is the major catalyst for fluorocarbon production from chlorocarbons. The most common chlorocarbon feedstocks for these reactions are carbon tetrachloride, ##STR1## and chloroform, ##STR2##
In the manufacturing process the reactants, for example, carbon tetrachloride and hydrogen fluoride, are bubbled through antimony pentachloride, a liquid, usually in a steel jacketed reactor. The products are removed continuously as volatile organics. The antimony pentachloride catalyst is not susceptible to catalyst poisoning, destruction, or even to serious processing losses. Instead, its activity is generally reduced in the system through simple dilution by by-products (e.g., tetrachloroethane) or by impurities in the feedstocks (e.g., methylene chloride in the chloroform feed and arsenic in the hydrogen fluoride feed). When the catalyst activity has been reduced below a practical level, the reactor mass is dumped and replaced by fresh, undiluted antimony pentachloride. The major components of the spent catalyst from fluorocarbon manufacturing are typically:
______________________________________ Antimony chlorides 35-45% Arsenic trichloride 5-10% Chloroform or carbon tetrachloride 20-30% 1,1,2,2, tetrachloroethane 10-20% Other halocarbons, e.g., chlorofluorocarbons 5-10% Hydrogen fluoride and hydrogen chloride 1-3% ______________________________________
The two impurities of greatest concern are arsenic and tetrachloroethane. Arsenic as arsenic trifluoride is an impurity present in the hydrogen fluoride, arsenic being generally present in the fluorspar from which hydrogen fluoride is derived. The amount of arsenic impurity present in the hydrogen fluoride thus varies from source to source. Tetrachloroethane as well as other C.sub.2 -C.sub.6 or higher boiling halocarbons or organic halogen compounds are formed during the fluorocarbon production process through undesirable side reactions.
The United States fluorocarbon industry generates approximately 500 metric tons of spent catalyst per year, but the significance is in the magnitude of the hazard and the potential for recovery and recycle, rather than in the annual volume of hazardous waste produced as such.
Several attempts have been made previously seeking to recover metal values from spent fluorocarbon catalysts. These attempts may be divided into two general categories: the recovery of antimony and/or arsenic from spent antimony catalyst in a non-recyclable catalyst form, and the recovery of antimony in a recyclable catalyst form.
U.S. Pat. Nos. 3,872,210 and 4,411,874 teach the general concept of extracting metals from spent catalysts into an aqueous phase. U.S. Pat. No. 3,872,210 teaches the use of aqueous acids or water to extract antimony while allowing recovery of oxide, hydroxide, sulfide or oxychloride antimony species. U.S. Pat. No. 4,411,874 discloses the use of CaCl.sub.2 solutions with recovery of antimony oxide, sulfide, hydroxide, and/or oxychloride salts. Arsenic separation is never considered nor discussed. neither patent appreciates the desirability of Sb(V) reduction to Sb(III) to allow for improved extraction. Nor does either patent disclose suitable methods for recovery of both antimony and arsenic values from the spent catalyst in a form appropriate for recycle to SbCl.sub.5 production and/or AsCl.sub.3, As.sub.2 O.sub.3 or arsenic metal production. Neither patent is concerned with the recovery of an incinerable organic stream.
U.S. Pat. Nos. 4,005,176 and 3,760,059 address the recovery of antimony values from spent catalyst in a form appropriate for catalyst recycle. Both patents are concerned with anhydrous systems and are not concerned with arsenic. U.S. Pat. No. 3,806,589 uses aqueous conditions but teaches Sb(V) reduction techniques, NH.sub.3 precipitation and distillation for SbCl.sub.3 recovery.
At the present time only limited technology exists to properly treat the extremely hazardous, toxic and potentially carcinogenic halocarbon wastes found in spent catalysts used in the production of fluorocarbons by means of a continuous contained process.